Structures and Spin States of Transition-Metal Structures and Spin States of Transition-Metal Cation Complexes with Aromatic LigandsCation Complexes with Aromatic Ligands

Free Electron Laser IRMPD SpectraFree Electron Laser IRMPD Spectra

Robert C. DunbarCase Western Reserve University

Structures and Spin States of Transition-Metal Structures and Spin States of Transition-Metal Cation Complexes with Aromatic LigandsCation Complexes with Aromatic Ligands

Coauthors:David T. Moore, Jos Oomens FOM Institute for Plasma Physics John R. Eyler Univ. of FloridaGerard Meijer, Gert von Helden Fritz Haber Institute

Free Electron Laser IRMPD SpectraFree Electron Laser IRMPD Spectra

Robert C. DunbarCase Western Reserve University

Structures and Spin States of Transition-Metal Structures and Spin States of Transition-Metal Cation Complexes with Aromatic LigandsCation Complexes with Aromatic Ligands

Spectroscopy of metal ion complexes

Cr+ ligand binding sites

Cr+ spin states (effect of coordination)

Transition metals with acetophenone

Rearrangement product ion structure

Infrared Spectroscopy of Molecular IonsInfrared Spectroscopy of Molecular Ions

Low densities of ionic molecules due to Coulombic repulsion:

Direct absorption spectroscopy difficult at best

Action spectroscopy, such as IR photodissociation Infrared multiple photon photodissociation = IRMPD

Free Electron Laser High powerExcellent tunability

Ion storage devicesTrap ions, irradiate for seconds

Free electron laserFree electron laser•

•Relativistic electron beam in periodic B field

• Wavelength determined by electron energy and B field

• High intensity pulsed output

FELIX – Free Electron Laser for Infrared eXperimentsFELIX – Free Electron Laser for Infrared eXperiments

Tuning range

Macropulse energy

Bandwidth

: 40 – 2000 cm-1

: 100 mJ

: Transform limited

Fourier Transform Ion Cyclotron Fourier Transform Ion Cyclotron Resonance Mass SpectrometerResonance Mass Spectrometer

4.7 T supercon magnet

~10 ppm homogeneity

Open ended ICR cell

Ion sources: EI, ESI, laser vaporization

Laser ablation metal ion source

Vapor phase complex formation

Moore, Oomens, van der Meer, von Helden, Meijer, Valle, Marshall & Eyler, Chem. Phys. Chem. 5, 740 (2004)

Generate complexes

Mass selective isolation

Irradiate with FELIX

Record MS

Plot fragment yield vs. IR wavelength

Density Functional Calculations of ComplexesDensity Functional Calculations of Complexes

Structure, Spin and Spectrum predictions

Choice of functional and basis set

Comparison of different spin states

Empirical scaling of vibrational frequencies

BSSE

Vibrational zero point energies and thermal energies

Binding Site:Binding Site:

Cr+(Aniline)

Choice of Ring (R) or nitrogen (N) binding sites

Cr+(Acetophenone)

Choice of Ring (R) or carbonyl oxygen (O) binding sites

CrCr++aniline: Ring Boundaniline: Ring BoundB3LYP: N-bound (E = 7.2 kJ/mol)

MP1PW91: ring-bound (E = 7.0 kJ/mol)

Oomens, Moore, von Helden, Meijer & Dunbar, JACS 126, 724 (2004)

CrCr++(acetophenone)(acetophenone)22: Side-chain Bound: Side-chain Bound

Spin StateSpin State

Cr+(Aniline) High spin

Cr+(Aniline)2 Low spin

High-spin sextet in weak ligand field

Low-spin doublet in strong ligand field

Cr+ is d5

CrCr++(aniline)(aniline)22: High or Low Spin ?: High or Low Spin ?

(S=5/2)

Exo Endo (Chelating)

O-Binding modesSpectroscopically similar

R BindingCharacteristic spectrum

MM++/Acetophenone Binding Modes/Acetophenone Binding Modes

400 600 800 1000 1200 1400 1600 1800 2000

cm-1

Co

Cr

Fe

Ni

Spectra of MSpectra of M++(Acet)(Acet)22

Cr and Ni very similar

Co distinctly different

Fe poorly resolved, not fully interpreted

400 600 800 1000 1200 1400 1600 1800 2000

cm-1

Co

Cr

Fe

Ni

Spectra of MSpectra of M++(Acet)(Acet)22

O bound

O bound

O bound and R bound

O bound and R bound

Ring umbrella

Side-chain stretchC=O stretch

400 600 800 1000 1200 1400 1600 1800 2000

cm-1

Co

Cr

Fe

Ni

Fits to CalculationsFits to Calculations

OO bound

OR bound

OR bound

OO bound

Summary: Transition metals with AcetophenoneSummary: Transition metals with Acetophenone

Ni similar to Cr: All ligands O bound

Co shows extra peaks: R bound and O bound ligands.Good fit to O/R complex (but mixtures possible)Special affinity of Co+ for benzene ring

Fe spectrum not as good: Clearly both O bound and R bound ligandsVarious possibilities

Characterizing a RearrangementCharacterizing a Rearrangement

Co+ active in bond activation chemistry

Look at the product of the reaction

Co+ + Acet Co+C7H8 + CO

Spectrum of CoSpectrum of Co++CC77HH88 Product Ion Product Ion

FELIX Spectrum

Co+(Toluene) Calculation

Rearrangement ProductRearrangement Product

The product ion spectrum fits the expected Co+

(Toluene) spectrum

Other possible product structures don’t fit

Reflects Co+ insertion into a C-C bond, rearrangement and coupling to form toluene, and expulsion of CO.

ConclusionsConclusions

FEL based IRMPD spectroscopy of trapped ionic species

Application to transition metal complexes gives valuable new insights

Structural characterization for ligands with competing binding sites

ring versus side-chain binding gives clear IR fingerprint

Spin state determination from vibrational spectrum is possible

ring-bound Cr+ bis-complexes are low spin

Spectra can characterize products of complex rearrangement reactions

Possible ConformationsPossible Conformations

Spin state: mono vs bis complexSpin state: mono vs bis complexCr+Anisole mono and bis complexes are ring bound.

Cr+(Anisole)1

high spin

Cr+(Anisole)2

low spin